A Japanese research group developed a hybrid SSD architecture using ReRAM (resistive random-access memory), which realizes high speed data rewriting, and a high capacity NAND flash memory.
The group is led by Ken Takeuchi, professor at the Department of Electrical, Electronic and Communication Engineering, the Faculty of Science and Engineering, Chuo University.
Compared with existing SSDs using only NAND flash memory, it can realize an 11 times higher data writing performance, 93% lower power consumption and 6.9 times longer product life, the group said. The new architecture will be announced at the 2012 Symposium on VLSI Circuits, which runs from June 13 to 15, 2012, in Hawaii, the US (lecture number: C-16.3).
In the case of NAND flash memory, the minimum unit used for writing data (page) is as large as 16 Kbytes, and it is not possible to overwrite data on the same page. Therefore, for a random access that overwrites 512 bytes (1 sector) of data, it is necessary to write 16 Kbytes of data in a different area and delete the original data later. When SSDs are used for servers at financial institutions, data writing performance lowers and power consumption increases because such a random access is dominant.
This time, the research group proposed a hybrid SSD that combines a 256-Gbyte NAND flash memory and about 8 Gbits of ReRAM used as both cache and storage memories. Because ReRAM enables a high-speed random access, it is possible not only to improve SSD performance and lower power consumption but also to drastically increase product life by using ReRAM for rewriting small data.
Specifically, data writing is controlled by using the following three algorithms. First, the AF (anti-fragment) algorithm writes small data in units of sectors in the ReRAM. Once about one page of data is stocked, it is written in the NAND flash memory. As a result, the frequency of random access to the NAND flash memory can be reduced. However, because the capacity of the ReRAM is limited, smaller data such as data that uses 60% of a page is sometimes written to the NAND flash memory, depending on the ReRAM's free space.
Second, to overwrite a small amount of data in the NAND flash memory after writing a page of data in it, the "RAAF (reconsider as a fragmentation)" algorithm transfers the page of data to the ReRAM again so that data is not fragmented in the NAND flash memory.
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